Estrategias de uso de recursos, resistencia y tolerancia a la remoción de biomasa aérea en plantas nativas del centro-oeste de Argentina

Abstract

La herbivoría es una fuerza estructuradora fundamental en los ecosistemas terrestres, que permite el ingreso de la productividad primaria a la red trófica. Además, mediante diferentes mecanismos, modula la productividad primaria y la dinámica de la materia y energía en los ecosistemas. Los caracteres foliares de las plantas también tiene un rol preponderante en esta dinámica. Dichos caracteres presentan un patrón de covariación que configura un eje de variación conocido como Espectro Económico Foliar (EEF) global y que da cuenta de la estrategia de uso de recursos de las plantas. El impacto más prominente de la herbivoría sobre los individuos vegetales es la remoción de biomasa, la cual puede afectar la aptitud del individuo y por lo tanto imponer una presión de selección. Se espera que intensidades contrastantes de herbivoría por ungulados domésticos generen diferencias a nivel intraespecífico, tanto en el EEF como en la tolerancia a la remoción de biomasa y en las defensas antiherbívoro. A su vez, los caracteres que configuran este espectro están relacionados por una estructura causal común, con compromisos bio-físicos y fisiológicos, por lo que se espera que la estructura de correlaciones se conserve a nivel intra-específico y que la herbivoría promueva cambios en todos ellos, de manera coordinada. Hasta la actualidad la información disponible respecto del EEF intra-específico y del efecto de la herbivoría sobre el mismo es escasa. Por lo que, en primer lugar, se abordó el estudio de las relaciones entre caracteres foliares a nivel intra-específico. Dada la relación a nivel global, entre el EEF y la tasa de crecimiento, la relación entre la tasa de crecimiento y la tolerancia, así como los potenciales compromisos entre tolerancia y defensas, se llevó a cabo un abordaje conjunto de las relaciones entre el EEF, tolerancia a la remoción de biomasa, las defensas anti-herbívoro y del efecto de la herbivoría por ungulados domésticos sobre todos estas variables.

Herbivory is a major structuring force on terrestrial ecosystems, driving the inputs of primary productivity to the whole trophic web. It also modulates primary productivity and the dynamics of matter and energy in the ecosystems by different mechanisms. Also, the plant functional traits related to the resource use strategy that configure the Leaf Economic Spectrum (LES) play an important role in this dynamics. Herbivory affects plant individuals mainly through biomass removals, which imposes fitness costs and often selection pressures. It is therefore expected that contrasting histories of herbivory by domestic ungulates should result in intra-specific differences in the LES, and also in the tolerance to biomass removal and in mechanical defenses such as spines. Because the traits that determine the LES are related by a common causal structure, with biophysical and physiological trade-offs, it is expected that the correlation structure should remain at the intra-specific level and that herbivory should promote coordinate changes in all traits. The available information to date about the effect of herbivory on the LES is scarce and has been the result of different frameworks. The information about the trade-offs that configure the LES at the intra-specific level is also scarce. Because of the global relationship between the LES and relative growth rate, the effect of herbivory by domestics ungulates on the LES, the tolerance to biomass removal (by cutting and by fire), and the mechanical defenses traits were analyzed together. The leaf traits of interest (LA, SLA, LDMC, Nm, C:N, FTF) were measured in six grass species and in four woody legume species, from sites with contrasting pressures of grazing and browsing during at least the last 40 years. Also, measurements of spines were taken in the woody species. Defoliation experiments were performed in these individuals in order to evaluate the plasticity of such traits and the tolerance to biomass removal. In addition, seeds from the ten species were grown in a common garden and leaf traits and spines were measured on their descendants. Finally, cutting and burning treatments were applied to analyze tolerance and the plasticity of the traits. The correlation patterns between the LES traits were found to be the same as at the intra-specific

level, as expected, suggesting that the same causal structure proposed in the literature for the inter- specific level applies at both levels. The intra-specific variability of such traits was high, with an

important plastic component, but also there is a hereditary component. In the grasses, herbivory induced plastic responses and selected for a more acquisitive syndrome, by changing the average value of traits. But herbivory also affected the relationships between traits, leading to changes in the orientation of the intra-specific LES. The effect of acquisitiveness on the tolerance to the biomass removal was positive, suggesting that phenotypic and trans-generationals changes are adaptive. Some effects of the history of herbivory on the LES were found in the woody species, but they did not show a clear direction. No plastic responses to cutting treatments were found. However, the individuals with severe history of herbivory were more tolerant to cutting and had more numerous and larger thorns. At the same time, it was found a positive relation between tolerance and spine size, which means that, in that case, there is no trade-off between tolerance and defenses as a resistance strategy against herbivory. A positive relation between tolerance to biomass removal by cutting and burning was also found. In conclusion, in the species set analysed, herbivory by domestic ungulates leads to phenotypic heritable differences, even in "ecological" (=short) time frames and local scales, selecting for higher tolerance to biomass removal. That tolerance is reached by a more acquisitive syndrome in the grasses but not in the woody legume species. In the latter, tolerance to herbivory leads to a higher tolerance to biomass removal by burning. Additionally, in the present study, the tolerance to herbivory is not in conflict with higher investment constitutive mechanical resistance traits.